The present invention relates to circularly polarized antennas, and more particularly, to such antennas that are used in television broadcasting.
Recently there has been much interest in using circular polarization for television broadcasting to improve reception and reduce ghosts in the displayed picture. To achieve this, various structures have been used, such as loops, dipoles, or coils, supported by a central vertical mast. However, such radiators increase the wind loading over what would be present with just the mast and also cause icing problems. One way to overcome these problems is to use as radiators slots that are cut into the mast. In order to achieve circular polarization, at least one pair of slots must be used which are spatially oriented 90.degree. with respect to each other and which are fed with equal amplitude signals that are 90.degree. in time phase with respect to each other. One way of achieving this is shown in U.S. Pat. No. 2,756,421, which shows pairs of slots, wherein the slots of any one pair are at right angles with respect to each other and have a vertical center to center spacing of a quarter of a wavelength of the excitation wave within the mast in order to achieve the 90.degree. time phasing. However, it has been found that the vertical component radiated by such an antenna will end fire radiate in a direction other than the desirable horizontal direction. Further, certain antennas have undesirable nulls in the elevation pattern and have beam tilt change and a poor impedance characteristic across a channel.
It is therefore desirable to have a circularly polarized antenna that has low windloading, a minimum of icing problems, does not radiate substantial energy in an end fire, have undesirable nulls, or have beam tilt change or a poor impedance characteristic across a channel.